Known indicated dosimeters include primarily ionization chambers and semiconductor sensors which, during the invasion of ionizing radiation into the sensor material, cause a absorbed dose proportional charge separation. The collected charge or the related discharge current can then be captured by an evaluation unit. The charge which has been collected over a period of time multiplied by a calibration factor gives the total absorbed dose.
In AT 008 309 U1 a method is described for the detection and counting of small electronically charged particles, by which a threshold value is increased if a measurement taken during a the beginning of a certain time period is too different from that taken at the end. In this way, it is possible to capture individual highly energetic particles in real time. However, the disadvantage with this is that it occurs at the expense of the accuracy of the measurement, which is extremely important, especially concerning therapeutic radiation equipment.
In DE 36 40 756 A1 a warning device is described for the detection of beams of ionizing radiation, by which the discharge of a capacitor, protected from the light and plugged into a corresponding low-voltage source, is captured. This kind of detection device is not suitable for the precision capture of x-rays from therapy equipment.
A dosimeter according to the preamble of claim 1 is described in U.S. Pat. No. 6,665,161 B1. The dosimeter is designed based on a particularly high sensitivity towards ionizing radiation, whereby the dosimeter includes an integrated field circuit with a built-in differential amplifier. However, this kind of dosimeter is not suitable for use in radiation therapy equipment, as the dominant radiation would cause the saturation of the dosimeter.
In DE 100 42 076 A1 a dosimeter according to the preamble of claim 1 is also described for the simultaneous determination of fast and thermal neutrons, as well as gamma radiation in personal dosimeters. The disadvantage of this dosimeter is that the sensor is so large that a beam with a small diameter, for example less than 3 cm, can only be roughly measured. With such small beam diameters, the absorbed dose rate depends so highly on a distance from the center of the beams that the absorbed dose rate on one side of the sensors differs significantly from the absorbed dose rate at another end of the sensor. The dosimeter captures an average so that an exact result concerning the absorbed dose rate at each point on the cross section of the beam is not possible.
Another disadvantage is that, with known dosimeters, the size of the dosimeter probe and also the sensor cannot be further reduced because it leads to significant measurement errors, due to following reasons: the smaller the sensor probe and therefore the sensor, the fewer the charges that are released from the ionizing radiation. Each dosimeter probe does however inevitably have a leakage current. In addition, parasitic ionization occurs in the radiation field. Both effects cause charges, which add to those charges which are to be lead directly back to the absorbed dose of the beam. The leakage current and the parasitic ionization do not decrease at the same rate depending on the size of the dosimeter probe, like the current resulting immediately from ionization. In this way, the smaller the dosimeter probe and therefore the sensor, the worse is the signal-to-noise ratio.